<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        /**
         * typeof区别
         */
        var a = null,
            b = undefined
        console.log(typeof a); // object
        console.log(typeof b); // undefined
    </script>
</body>
</html>